Abstract. A method has been developed for using particle hygroscopicity measurements
made with a humidified tandem differential mobility analyzer (HTDMA) to
determine water activity as a function of solute weight percent. In Part I,
the method was tested for particles composed of sodium chloride and ammonium
sulfate. Here, we report results for several atmospherically-relevant
organic species: glutaric acid, malonic acid, oxalic acid and levoglucosan.
Predicted water activities for aqueous dicarboxylic acid solutions are quite
similar in some cases to published estimates and the simplified predictions
of Köhler theory, while in other cases substantial differences are
found, which we attribute primarily to the semivolatile nature of these
compounds that makes them difficult to study with the HTDMA. In contrast,
estimates of water activity for levoglucosan solutions compare very well
with recently-reported measurements and with published data for aqueous
glucose and fructose solutions. For all studied species, the critical dry
diameters active at supersaturations between 0.2 and 1% that are computed
with the HTDMA-derived water activities are generally within the
experimental error (~20%) estimated in previously-published direct
measurements using cloud condensation nuclei counters. For individual
compounds, the variations in reported solution water activity lead to
uncertainties in critical dry diameters of 5-25%, not significantly
larger than the uncertainty in the direct measurements. To explore the
impact of these uncertainties on modeled aerosol-cloud interactions, we
incorporate the variations in estimates of solution water activities into
the description of hygroscopic growth of aerosol particles in an adiabatic
parcel model and examine the impact on the predicted drop number
concentrations. For the limited set of initial conditions examined here, we
find that the uncertainties in critical dry diameters for individual species
lead to 0-21% changes in drop number concentration, with the largest
effects at high aerosol number concentrations and slow updraft velocities.
Ammonium sulfate, malonic acid and glutaric acid have similar activation
behavior, while glutaric acid and levoglucosan are somewhat less hygroscopic
and lead to lower drop number concentrations; sodium chloride is the most
easily activated compound. We explain these behaviors in terms of a
parameter that represents compound hygroscopicity, and conclude that this
parameter must vary by more than a factor of 2 to induce more than a 15%
change in activated drop number concentrations. In agreement with earlier
studies, our results suggest that the number concentration of activated
drops is more sensitive to changes in the input aerosol size and number
concentrations and the applied updraft velocity than to modest changes in
the aerosol composition and hygroscopic properties.